In recent times, use of gas discharge lamps instead of incandescent bulbs is growing as a result of their high efficiency. In terms of operation, high pressure discharge lamps are more difficult to handle than low pressure discharge lamps in this case, and the electronic operating devices for these lamps are therefore more expensive.
High pressure discharge lamps are usually operated by means of a low-frequency square-wave current, also known as intermittent direct current operation. In this case, an essentially square-wave current having a frequency of usually 50 Hz to several kHz is applied to the lamp. The lamp commutates with each oscillation between positive and negative voltage, because the current direction also changes and the current is therefore briefly at zero. This operation ensures that the electrodes of the lamp are uniformly loaded in spite of quasi-direct current operation.
Gas discharge lamps are successfully used for display systems, for example, because they can generate a high luminance which can be subsequently processed by an inexpensive lens system. Display systems and their lighting apparatus are described in the publications U.S. Pat. Nos. 5,633,755 and 6,323,982, for example. Display systems such as DLP projectors (DLP: digital light processing) include a lighting apparatus having a light source whose light is directed onto a DMD chip (DMD: digital mirror device). The DMD chip microscopically includes small tilting mirrors, which either direct the light onto the projection surface if the associated pixel is to be turned on, or direct the light away from the projection surface, e.g. onto an absorber, if the associated pixel is to be switched off. Each mirror therefore acts as a light valve which controls the light level of a pixel. These light valves are generally known as DMD light valves. For the purpose of generating colors in the case of a lighting apparatus which emits white light, a DLP projector includes a filter wheel, for example, which is arranged between lighting apparatus and DMD chip and contains filters of various colors, e.g. red, green and blue. By means of the filter wheel, light of the currently desired color is sequentially transmitted from the white light of the lighting apparatus.
The color temperature of such display systems is normally dependent on the spectrum locus of the light of the lighting apparatus. This usually changes according to the operating parameters of the light sources of the lighting apparatus, e.g. voltage, current intensity and temperature. Furthermore, depending on the light sources used in the lighting apparatus, the ratio between current intensity and light level is not necessarily linear. Consequently, a change of the current intensity also results in a change of the spectrum locus of the light of the light source, and hence in a change of the color temperature of the display system.
Furthermore, the color depth of the display system is limited by the minimal ON-time of a pixel. In order to increase the color depth, it is possible to implement e.g. dithering, wherein individual pixels are switched using a lower frequency than the regular frequency of 1/60 Hz. However, this usually results in noise which is visible to a human observer.
The contrast ratio of the display system is defined by the ratio of the maximal light level resulting from fully opened light valves to minimal light level resulting from fully closed light valves. In order to increase the contrast ratio of a display system, the minimal light level resulting from fully closed light valves can be further reduced by means of a mechanical screen, for example. However, a mechanical screen requires space in the lighting apparatus or the display system, increases the weight of the lighting apparatus or the display system, and also represents an additional potential source of interference. High pressure discharge lamps such as those used in such display systems can also be operated in a dimmed mode, though the dimmed operating mode raises problems with regard to the electrode temperature and the arc root in the high pressure discharge lamp.
The arc root is generally problematic when alternating current is used for operation of a gas discharge lamp. When alternating current is used for operation, a cathode becomes an anode and an anode conversely becomes a cathode during commutation of the operating voltage. The cathode-anode transition is not problematic in principle, since the temperature of the electrode does not have any effect on its anodic operation. In the case of the anode-cathode transition, the ability of the electrode to supply a sufficiently high current is dependent on its temperature. If this is too low, the electric arc changes during the commutation, usually following a zero crossing, from a concentrated arc root operating mode to a scattered arc root operating mode. This change is accompanied by an interruption in the light output, which is often visible and can be perceived as flickering.
Ideally therefore, the lamp is operated in concentrated arc root operating mode, since the arc root in this case is very small and therefore very hot. As a consequence of this, less voltage is required here due to the higher temperature at the small root point, in order to be able to supply sufficient current. An electrode tip which has a uniform shape and whose surface is not fissured supports the concentrated arc root operating mode and hence safer and more reliable operation of the gas discharge lamp.
In the following, commutation is considered to be the process in which the polarity of the voltage of the gas discharge lamp alternates, and in which a significant change in current or voltage therefore occurs. In the case of an essentially symmetrical operating mode of the lamp, the voltage zero or current zero occurs in the middle of the commutation time. It should be noted in this context that the voltage commutation usually always occurs more quickly than the current commutation.
The inner end of the lamp electrode, said inner end projecting into the discharge space of the gas discharge lamp burner, is referred to below as an electrode end. A needle or peak-shaped raised part which is positioned on the electrode end, and whose end is used as a root point for the electric arc, is referred to as an electrode tip.
The variation or distortion of the electrodes over the entire service life represents a significant problem of high pressure discharge lamps. In this case, the shape of the electrode changes from the ideal shape to an increasingly fissured surface, particularly at the inner end of the electrode. Moreover, there is a risk of producing electrode tips that are not arranged in the center of the relevant electrode. The discharge arc always forms from electrode tip to electrode tip. If a plurality of electrode tips of approximately equal validity are present on an electrode, this can result in arc jumping and hence to flickering of the lamp. Electrode tips which grow non-centrically will degrade the optical image, since the lens system of a projector or a light (in which such a discharge lamp is installed) is configured relative to a specific position of the discharge arc, and in particular is adjusted relative to the initial state of the electrodes and the discharge arc. In certain cases, the electrode tips can grow unevenly, such that the electric arc is no longer arranged centrally in the burner vessel, but is shifted axially. This likewise degrades the optical image of the overall system. By contrast, the fissuring results in an increase of the original electrode separation and therefore also affects the lamp voltage. As this increases proportionally relative to the separation, it can result in premature service life shutdown, since this usually occurs when the lamp voltage exceeds a predetermined threshold value. In summary, this results in a reduction in the lamp service life and in the quality of the light emitted from the lamp.
The prior art does not currently disclose any solutions to these problems. Merely for the sake of completeness, reference is made to WO 2007/045599 A1. While the problem giving cause to the present invention occurs at the end of the lamp service life, the cited publication addresses a problem which occurs within the first three hundred operating hours. Tip growth can occur during this period, resulting in a reduction of the electrode separation. This causes the lamp voltage to decrease, such that the current to be supplied by an electronic operating device must be increased in order to achieve a constant power. Since electronic operating devices are naturally configured for a specific maximum current, this results in problems. In order to avoid an increase of the current configuration for the continuous operation and the resulting occurrence of additional costs, the cited publication proposes that a current pulse be applied to the electrodes, such that the electrode tips which have grown are fused back. In this way, the separation of the electrodes can be increased again, the lamp voltage increased, and the required current therefore decreased. By contrast, however, the present invention addresses the problem of conserving the electrodes in an optimal state, as far as possible over the entire service life of the gas discharge lamp, wherein the electrodes have a relative separation which corresponds as far as possible to the original separation that is present in a new lamp, and wherein the surface of the electrode ends remains smooth and has tips which grow centrically, forming a defined root point for the arc. The teaching of WO 2007/045599 A1 does not therefore solve the problem cited above.